I've been doing some research into brake fluid and it seems there's not a lot to worry about providing you use the same type of fluid:
DOT3, DOT4 & DOT 5.1 are all Glycol-based.
Glycol is Hygroscopic - is absorbs moisture.
Having water in the fluid lowers its boiling point and increases the risk of brake fade due to vapour lock if the brakes are used hard.
Glycol brake fluid is rated by two boiling point values - 'Dry' (no water) and 'Wet' (with a standard of 3.7% absorbed water).
The difference between DOT3, DOT4 & DOT5.1 is in their boiling points:
Characteristics of common braking fluids
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| Dry boiling point | Wet boiling point[a] | Viscosity at -40 °C°F | Viscosity at 100 °C (212 °F) | Primary constituent |
---|
DOT 2 | 190 °C (374 °F) | 140 °C (284 °F) | ? | ? | castor oil/alcohol |
DOT 3 | 205 °C (401 °F) | 140 °C (284 °F) | ≤ 1500 mm2/s | ≥ 1.5 mm2/s | glycol ether |
DOT 4 | 230 °C (446 °F) | 155 °C (311 °F) | ≤ 1800 mm2/s | ≥ 1.5 mm2/s | glycol ether/borate ester |
DOT 4+ | 230 °C (446 °F) | 155 °C (311 °F) | ≤ 750 mm2/s | ≥ 1.5 mm2/s | glycol ether/borate ester |
LHM+ | 249 °C (480 °F) | 249 °C (480 °F) | ≤ 1200 mm2/s[14] | ≥ 6.5 mm2/s | mineral oil |
DOT 5 | 260 °C (500 °F) | 180 °C (356 °F) | ≤ 900 mm2/s | ≥ 1.5 mm2/s | silicone |
DOT 5.1 | 260 °C (500 °F) | 180 °C (356 °F) | ≤ 900 mm2/s | ≥ 1.5 mm2/s | glycol ether/borate ester |
DOT 5.1 ESP | 260 °C (500 °F) | 180 °C (356 °F) | ≤ 750 mm2/s | ≥ 1.5 mm2/s | glycol ether/borate ester |
- ^ "Wet" defined as 3.7% water by volume
There are other categories of brake fluid that we don't usually encounter.
The main alternative is DOT5 - this is silicon-based, cannot be mixed with Glycol and is coloured blue/purple to differentiate it from the pale brown/straw Glycol-based fluid.
Silicone is Hydrophobic - it repels water. I'm surprised to see a 'Wet' value in the table above as it won't have any absorbed moisture - though there may be free water in the brake line I suppose.
Glycol fluid should be replaced every 2-3 years. Silicone fluid can be left longer.
Silicone is used where brakes are likely to be very hot - racing cars & bikes - and by the Military who don't change their hydraulic fluid regularly if possible.
One interesting comment (I won't call it a fact) I turned-up is that Silicone fluid is more compressible than Glycol fluid - therefore the brake pedal will feel a little 'spongy' if Silicone fluid is used.
Glycol fluid contains 'Corrosion Inhibitor' to reduce the risk of corrosion in the braking system. No-where did I find anything that said that DOT3, DOT4 or DOT5.1 had different inhibitors and therefore couldn't be mixed.
It appears from what I've read and seen on you-tube vids is that DOT3, DOT4 & DOT5.1 can be mixed.
I'm working on a 2007 Mazda6. The Haynes Manual says use DOT3 fluid. I'll be flushing it through with DOT4 as that's what I use on all the other cars & motorcycles. I don't expect any problems.
Bob.